(1) Field of the Invention
The present invention relates to a projection type image display device.
(2) Description of the Related Art
In a projection type image display device that projects an image at a given angle (for example, from a lower side of a screen) with respect to an oblique projection, that is, a normal to a screen principal plane, an aspherical mirror or a free-shaped surface mirror is employed in an outgoing optical part of a projection optical system. In general, those mirrors are manufactured by molding with a resin and depositing aluminum or silver, taking mass productivity into consideration. However, the lenses are liable to be charged, and therefore liable to be contaminated with the dust or dirt in air. When the dust or dirt that is attached to the mirror surface is wiped off, hair line shaped scratches are produced on the mirror surface due to components contained in the dust or dirt which are harder than a coating material of the mirror surface. There arise such disadvantages that not only those scratches induce a deterioration of the image contrast, but also moisture enters the mirror from the scratches to rust the deposited film of the mirror.
In order to cope with the above problems, Japanese Patent Application Laid-Open Publication No. 2002-082387 discloses a technique in which a projection optical system is housed in a casing with a window, and a transparent plane plate that is formed of a glass plate is disposed between the projection optical system and the screen to prevent the mirror surface from being contaminated by the dust or dirt.